Data from researchers in Israel, which has inoculated swathes of its population, suggests the Pfizer/BioNTech vaccine is reducing viral load, a key signal that the intervention could diminish the spread of Covid-19.
Evidence that the coronavirus vaccines being deployed globally are dramatically effective in reducing severe disease and death in symptomatic Covid-19 is abundant. But a big question remains unanswered: can they thwart transmission, in other words stop people from passing on the virus?
Preventing the spread of infection is key to reducing the risk of more variants emerging and to achieving herd immunity, scientists say. People with higher viral load tend to be more infectious and are more likely to suffer from severe disease.
To evaluate the impact of the vaccine on transmission, researchers compared data from people over 60 years old and those aged 40 to 60, evaluating data from 16,297 people who had tested positive for coronavirus between 1 December and 30 January.
Israel’s vaccine programme began on 20 December. By the time of analysis, more than 75% of those in the older group were likely to have received their first dose, as had about 25% of the younger group.
Although researchers did not know whether each person had been given their first vaccine dose, their hypothesis was that if the vaccine was reducing viral load, then evidence of that would begin to show up in late January but not before, because of the time required by the vaccine to stimulate the immune system.
As expected, in the last two weeks of January the researchers noted a statistically significant fall in the viral load for the individuals aged over 60, compared with the 40-to-60 group, said the study author, Yaniv Erlich, the chief scientific officer of MyHeritage, a company that runs a large coronavirus testing laboratory in Israel.
The researchers used available demographic data and vaccination rates to estimate the effect of the first dose on viral load reduction, calculating that the vaccine reduced the viral load by 1.6 to 20 times in individuals who tested positive for the virus.
Erlich cautioned that the paper, which has yet to be peer-reviewed or published in a medical journal, was just an initial study. While it was understood that a smaller viral load was better, it was not clear whether this reduction would be enough to block transmission. “My expectation is … that if you are positive following vaccination, you probably will transmit the disease to a smaller number of people, on average,” he said.
Stephen Griffin, associate professor at Leeds University’s school of medicine, said the data, though early, was positive but not definitive. Since the study looked at people who had tested positive for the virus, the results suggested that vaccinated people were less likely to pass infection on, potentially, by virtue of the fact they had a reduced virus load, he said.
It remained unclear whether the vaccine stopped people from getting infected altogether – however, this study was not designed to assess that, Griffin said.
Last week, Oxford/AstraZeneca unveiled a preliminary analysis from a small subset of patients that hinted that one dose of their vaccine could cut transmission of the virus by up to 67%, on the basis of a reduction in viral load.